Improved Estimation of Protein Rotational Correlation Times from 15N Relaxation Measurements.

In the study of protein backbone dynamics by 15N relaxation measurements, an initial estimation of the isotropic global correlation time, taum, is usually obtained from the average T1/T2 ratio of nuclear spins that do not exhibit slow internal motion and with T2 values not significantly shortened by chemical or conformational exchange processes. Different methods have been used for identification of the rates of internal motion. However, the number of nuclear spins included in the taum estimation is often larger than the number that ultimately can be fitted to a single-order parameter, S2, implying that some nuclear spins involved in the initial taum estimation actually have an effective internal correlation time, taue, not as fast as assumed. As a consequence, taum is underestimated, since internal motion reduces the T1/T2 ratio. This situation becomes more obvious if the molecule has a large taum value because the reduction in T1/T2 ratio arising from internal motion is more significant than for molecules with smaller taum and the same degree of internal motion. This Communication describes a more reliable method for identifying nuclear spins which should be excluded from the taum estimation because of insufficiently rapid internal motion. This results in an improved taum value, giving a much better agreement between the number of nuclear spins fitted successfully to a single-order parameter, S2, and those used in the taum estimation. Copyright 1998 Academic Press.